Vari-focal spectacles

ABSTRACT

Optical elements such as a vari-focal lens element, a vari-focal diffractive optical element and a variable declination prism usable as spectacle lens elements and so on.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to optical elements such as a vari-focallens element, a vari-focal diffractive optical element and a variabledeclination prism which are to be used as liquid crystal opticalelements. The present invention also relates to an electronic imagepickup unit which uses these optical elements.

b) Description of the Prior Art

For composing a vari-focal lens system of lens elements which aremanufactured by polishing a glass material, it is conventional to changea focal length of the lens system by moving a lens unit(s) in adirection along an optical axis, for example, as in a zoom lens systemfor cameras since the lens elements cannot change focal lengths bythemselves. However, such a lens system has a drawback that it has acomplicated mechanical structure.

For correcting such a drawback, there has been proposed an opticalsystem which uses a polarizing plate 1 and a liquid crystal lenscomponent 2, for example, as shown in FIG. 1. The liquid crystal lenscomponent 2 comprises lens elements 3 a and 3 b, and a liquid crystallayer 5 which is disposed between these lens elements by way oftransparent electrodes 4 a and 4 b, and an AC power source 7 isconnected between the transparent electrodes 4 a and 4 b by way of aswitch 6, whereby the optical system is configured to change arefractive index of the liquid crystal layer 5 by selectively applyingan electric field to the liquid crystal layer 5.

When natural light, for example, is incident on the polarizing plate 1of this optical system, only a predetermined linearly polarizedcomponent transmits through the polarizing plate 1 and is incident onthe liquid crystal lens component 2. In a condition where the switch 6is turned off and no electric field is applied to the liquid crystallayer 5 as shown in FIG. 1, longer axes of liquid crystal molecules 5 aare oriented in a direction of a plane of polarization of the incidentlinearly polarized component, whereby a refractive index of the liquidcrystal layer 5 is enhanced and a focal length of the liquid crystallens component 2 is shortened. In a condition where the switch 6 isturned on and an electric field is applied to the liquid crystal layer 5as shown in FIG. 2, in contrast, the longer axes of the liquid crystalmolecules 5 a are oriented in parallel with an optical axis, whereby therefractive index of the liquid crystal layer 5 is lowered and the focallength of the liquid crystal lens component 2 is prolonged. The focallength of the optical system shown in FIG. 1 is variable by selectivelyapplying an electric field in the liquid crystal lens component 2 asdescribed above.

However, the optical system shown in FIG. 1 poses a problem that itattenuates rays to be incident on the liquid crystal lens component 2during transmission through the polarizing plate 1 and lowers a lightutilization efficiency since it requires to dispose the polarizing plate1 before the liquid crystal lens component 2 so that only thepredetermined linearly polarized component is incident on the liquidcrystal lens component 2. Further, the optical system which utilizeslight at such a low efficiency poses another problem that it isapplicable only to limited instruments or has a low versatility.

Further, an electronic image pickup unit for electronic cameras, videocameras and the like consists of a combination of an image pickup device8 and a lens system 9 as shown in FIG. 3.

Such an electronic image pickup unit generally uses a lens system whichhas a relatively complicated composition, has a complicatedconfiguration as a whole, comprises a large number of parts and requirestedious assembly, thereby being limited in compact design and reductionof a manufacturing cost thereof.

SUMMARY OF THE INVENTION

In view of the conventional problems described above, a primary objectof the present invention is to provide optical elements having variableoptical characteristics, i.e., a vari-focal optical element, avari-focal diffractive optical element, a vari-focal mirror and avariable declination prism usable as liquid crystal optical elementswhich are adequately configured so as to enhance light utilizationefficiencies, be applicable efficiently to various kinds of opticalinstruments and has excellent versatility.

The vari-focal optical element according to the present invention ischaracterized in that it comprises: a first optical member which hasfirst and second surfaces, and allows incident rays to transmit throughthe first and second surfaces; a second optical member having a thirdsurface which receives rays having transmitted through the first opticalmember; a lens surface which is formed on at least one of the first,second and third surfaces; a pair of transparent electrodes disposed onthe second surface and the third surface respectively; and a polymerdispersive liquid crystal layer which is disposed between thesetransparent electrodes, and that it is configured so as to be capable ofchanging a focused point of rays which have transmitted through thefirst and second optical members or rays which have transmitted throughthe first optical member, have been reflected by the third surface andhave transmitted again through the first optical member by applying anelectric field to the polymer dispersive liquid crystal layer by way ofthe pair of transparent electrodes.

Further, the vari-focal diffractive optical element according to thepresent invention is characterized in that it comprises: a first opticalmember which has first and second surfaces, and allows incident rays totransmit through the first and second surfaces; a second optical memberwhich has third and fourth surfaces, and allows rays which havetransmitted through the first optical member to emerge through the thirdand fourth surfaces; a diffractive surface which is formed at least oneof the first, second and third surfaces; transparent electrodes whichare disposed on sides of the second surface and the third surfacerespectively; and a polymer dispersive liquid crystal layer which isdisposed between these transparent electrodes, and that it is configuredso as to be capable of changing a focused point of rays which havetransmitted through the first and second optical elements by applying anelectric field to the polymer dispersive liquid crystal layer by way ofthe pair of transparent electrodes.

Furthermore, the variable declination prism according to the presentinvention is characterized in that it comprises: a first optical memberwhich has first and second surfaces, and allows incident rays totransmit through the first and second surfaces; a second optical memberhaving third and fourth surfaces, and allows rays which have transmittedthrough the first optical member to pass through the third and fourthsurfaces; an inclined surface which is formed on at least one of thefirst, second and third surfaces; transparent electrodes which aredisposed on sides of the second surface and the third surfacerespectively; and a polymer dispersive liquid crystal layer which isdisposed between these transparent electrodes, and that it is configuredso as to be capable of changing declinations of rays which havetransmitted through the first and second optical members by applying anelectric field to the polymer dispersive liquid crystal layer by way ofthe pair of transparent electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view illustrating a composition of aconventional optical system which uses a liquid crystal lens component;

FIG. 2 shows a sectional view illustrating a condition where an electricfield is applied to the liquid crystal lens component shown in FIG. 1:

FIG. 3 shows a sectional view illustrating a composition of aconventional electronic image pickup unit;

FIG. 4 shows a sectional view illustrating a theoretical composition ofthe vari-focal lens component according to the present invention;

FIG. 5 shows a diagram illustrating an optical indicatrix of a uniaxialnematic liquid crystal molecule;

FIG. 6 shows a sectional view illustrating a condition where an electricfield is applied to a polymer dispersive liquid crystal layer of thevari-focal lens component shown in FIG. 4;

FIG. 7 shows a sectional view illustrating a composition to vary avoltage applied to polymer dispersive layer of the vari-focal lenscomponent shown in FIG. 1;

FIG. 8 shows a sectional view exemplifying a digital camera which usesthe vari-focal lens component according to the present invention;

FIG. 9 shows a sectional view exemplifying an objective lens system forelectronic endoscopes which uses the vari-focal lens component accordingto the present invention;

FIG. 10 shows a sectional view illustrating an example of the vari-focaldiffractive optical element according to the present invention;

FIG. 11 shows a sectional view illustrating vari-focal spectacles whichuses the vari-focal diffractive optical element according to the presentinvention;

FIG. 12 shows a sectional view illustrating a condition wherein anelectric field is applied to the diffractive optical element of thevari-focal spectacles shown in FIG. 11;

FIG. 13 shows a perspective view illustrating spectacles which uses aconventional lens components having dual focal points;

FIG. 14 shows a diagram illustrating a modification example of thevari-focal spectacles;

FIG. 15 shows a perspective view illustrating another modificationexample of the vari-focal spectacles;

FIG. 16 shows a sectional view illustrating vari-focal spectacles havingvari-focal lens components which use a twist nematic liquid crystal;

FIG. 17 shows a sectional view illustrating an orientation of liquidcrystal molecules in a condition where a voltage applied to a twistnematic liquid crystal layer is enhanced in the spectacles shown in FIG.16;

FIG. 18 shows a perspective view illustrating connection betweenvari-focal spectacle lens components of the vari-focal spectaclesaccording to the present invention and a driving unit;

FIG. 19 shows a perspective view illustrating an overall configurationof the vari-focal spectacles according to the present inventionincluding the driving unit and so on;

FIG. 20 shows a perspective view illustrating a condition where a personputs on the vari-focal spectacles according to the present invention;

FIG. 21 shows a perspective view illustrating a condition where a personputs on another vari-focal spectacles according to the presentinvention;

FIG. 22 is a perspective view showing an example wherein a driving unitis disposed in a vari-focal lens component;

FIG. 23 is a perspective view showing an example wherein a drivingelectronic circuit is disposed in a vari-focal lens component;

FIG. 24 is a diagram showing an example to form a vari-focal lenscomponent so as to match with a spectacle frame;

FIG. 25 is a sectional view showing another example of driving circuitfor a vari-focal lens component;

FIG. 26 is a perspective view showing another example of connectionbetween vari-focal lens components of vari-focal spectacles and adriving unit;

FIGS. 27A and 27B are sectional views exemplifying the variabledeclination prism according to the present invention;

FIG. 28 is a sectional view showing a condition where the variabledeclination prism shown in FIGS. 27A and 27B is used;

FIG. 29 is a sectional view illustrating the vari-focal mirror accordingto the present invention;

FIG. 30 is a perspective view illustrating a radial gradientheterogeneous medium lens element;

FIG. 31 is a diagram illustrating a refractive index distribution of theradial gradient heterogeneous medium lens element;

FIG. 32 is a perspective view illustrating a material of a heterogeneousmedium lens element;

FIG. 33 is a diagram descriptive of a step to grind a heterogeneousmedium material with a centerless grinder;

FIG. 34 is a perspective view descriptive of a step to cut theheterogeneous medium material with a cutter;

FIG. 35 is a perspective view illustrating a condition where a cutheterogeneous medium material is bonded to a V block;

FIG. 36 is a diagram illustrating a condition where the heterogeneousmedium material is bonded to a gear;

FIG. 37 is a sectional view descriptive of a step to grind theheterogeneous medium material with a surface grinder;

FIG. 38 is a perspective view descriptive of a step to precisely grindand polish the heterogeneous medium material with a polishing machine;

FIG. 39 is a perspective view descriptive of a step to chamfer theheterogeneous medium material with an engine lathe;

FIG. 40 is a perspective view descriptive of another example of step toprecisely grind and polish the heterogeneous medium material with apolishing machine;

FIG. 41 is a diagram descriptive of a step to grind the heterogeneousmedium material with a sider type centering machine;

FIG. 42 is a diagram descriptive of a step to form a fixture to be usedfor manufacturing a heterogeneous medium lens element having sphericalsurfaces;

FIG. 43 is a sectional view showing a condition where the heterogeneousmedium material is fitted into the fixture;

FIG. 44 is a sectional view descriptive of a step to precisely grind andpolish the heterogeneous medium material with a polishing machine;

FIG. 45 is a diagram descriptive of a step to bond the heterogeneousmedium material to the fixture;

FIG. 46 is a diagram descriptive of a step to grind a curved surface ofthe heterogeneous medium material with a curve generator;

FIG. 47 is a sectional view descriptive of a step to grind an outercircumference of the heterogeneous medium material with a bell clampcentering machine;

FIG. 48 is a sectional view descriptive of a step to grind the outercircumference of the heterogeneous medium material after both surfacesthereof are ground;

FIG. 49 is a diagram descriptive of a step to grind a surface of theheterogeneous medium material with a curve generator;

FIG. 50 is a diagram descriptive of a step to grind the other surface ofthe heterogeneous medium material with a curve generator;

FIG. 51 is a sectional view illustrating a first embodiment of theelectronic image pickup unit according to the present invention;

FIG. 52 is a perspective view illustrating a second embodiment of theelectronic image pickup unit according to the present invention;

FIG. 53 is a sectional view illustrating a viewfinder section of thesecond embodiment of the electronic image pickup unit according to thepresent invention;

FIG. 54 is a sectional view illustrating a third embodiment of theelectronic image pickup unit according to the present invention;

FIG. 55 is a sectional view illustrating an optical element to be usedin the third embodiment of the electronic image pickup unit according tothe present invention;

FIG. 56 is a sectional view illustrating a condition of liquid crystalmolecules when an electric field is applied to a liquid crystal layer ofan optical element;

FIG. 57 is a sectional view illustrating a modification example of theoptical element to be used in the electronic image pickup unit accordingto the present invention;

FIG. 58 is a sectional view illustrating a fourth embodiment of theelectronic image pickup unit according to the present invention;

FIG. 59 is a sectional view illustrating a vari-focal Fresnel mirror tobe used in the fourth embodiment of the electronic image pickup unitaccording to the present invention;

FIG. 60 is a sectional view exemplifying application of a vari-focaldiffractive optical element; and

FIG. 61 is a sectional view illustrating another modification example ofthe optical element to be used in the third embodiment of the electronicimage pickup unit according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 is a sectional view illustrating a theoretical composition of thevari-focal lens component according to the present invention. Avari-focal lens component 11 comprises, in order from a side ofincidence of rays, a first lens element 12 a which has first and secondsurfaces 8 a and 8 b, a second lens element 12 b which has third andfourth surfaces 9 a and 9 b, and a polymer dispersive liquid crystallayer 14 which is disposed between these lens elements by way oftransparent electrodes 13 a and 13 b: the vari-focal lens elementfunctioning to converge incident rays with the first and second lenselements 12 a and 12 b. The transparent electrodes 13 a and 13 b areconnected to an AC power source 16 by way of a switch 15 so as to applyan AC electric field selectively to the polymer dispersive liquidcrystal layer 14. The polymer dispersive liquid crystal layer 14comprises a large number of minute polymer cells 18 which have optionalforms such as spheres and polyhedrons, contain liquid crystal moleculesrespectively, and has a volume which is made coincident with a sum ofvolumes occupied by polymers and liquid crystal molecules 17 composingthe polymer cells 18.

When the polymer cells 18 are spherical, for example, they are composedso as to satisfy, for example, the following condition (1):

2 nm≦D≦λ/5  (1)

wherein the reference symbol D represents a mean diameter of the polymercells 18 and the reference symbol λ designates a wavelength of lightused.

Since the liquid crystal molecules 17 have sizes on the order of 2 nm orlarger, the condition (1) defines a lower limit of the mean diameter Das 2 nm or larger. An upper limit of D is dependent on a thickness t ofthe polymer liquid crystal layer 14 as measured in the direction alongan optical axis of the vari-focal lens component 11. When D is large ascompared with λ, however, rays are scattered by border surfaces of thepolymer cells 18 due to a difference between a refractive index of thepolymers and that of the liquid crystal molecules 17 and the polymerdispersive liquid crystal layer 14 is opaque. It is therefore desirablethat D has a value not exceeding λ/5. When an optical instrument whichis to use the vari-focal lens component does not require so highprecision, it is sufficient that D has a value of x or smaller. In otherwords, it is sufficient that D satisfies the following condition (1-1):

2 nm≦D≦λ  (1-1)

Transparency of the polymer dispersive liquid crystal layer 14 is loweras the thickness t becomes larger.

Uniaxial nematic liquid crystal molecules, for example are used as theliquid crystal molecules 17 though it is possible to use various kindsof liquid crystals such as nematic liquid crystals, ferroelectric liquidcrystals, choresteric liquid crystals, discotie liquid crystals,diselectric liquid crystals and tolane liquid crystals. The liquidcrystal molecule 17 has an optical indicatrix having such a shape asthat shown in FIG. 5 to which the following formula (2) applies:

n _(0x) =n _(0y) =n ₀  (2)

wherein the reference symbol n₀ represents a refractive index of theordinary rays, and the reference symbols n_(0x) and n_(0y) designaterefractive indices in directions perpendicular to each other in a planeincluding the ordinary rays.

In a condition where the switch 15 is turned off as shown in FIG. 4,i.e., an electric field is not applied to the polymer dispersive liquidcrystal layer 14, the liquid crystal molecules 17 are set in variousdirections, whereby the polymer dispersive liquid crystal layer 14 has ahigh refractive index for incident rays and the vari-focal lenscomponent 11 functions as a lens component having a strong refractivepower. When an electric field is applied to the polymer dispersiveliquid crystal layer 14 by turning on the switch 15 as shown in FIG. 6,the liquid crystal molecules 17 are oriented so that longer axes of theoptical indicatrices are in parallel with the optical axis of thevari-focal lens component 11, whereby the polymer dispersive liquidcrystal layer 14 has a low refractive index and the vari-focal lenscomponent 11 functions as a lens having a weak refractive power.

A voltage applied to the polymer dispersive liquid crystal layer 14 canbe varied stepwise or continuously as shown in FIG. 7, for example, witha variable resistor 19. By varying the voltage as described above, it ispossible to vary a refractive power stepwise or continuously since theliquid crystal molecules 17 are oriented so that the longer axes of theindicatrices are progressively in parallel with the optical axis of thevari-focal lens component 11 as the applied voltage becomes higher.

In the condition shown in FIG. 4 where an electric field is not appliedo the polymer dispersive liquid crystal layer 14, a mean refractiveindex n_(LC)′ of the liquid crystal molecules 17 is approximatelyexpressed by the following equation (3):

(n _(0x) +n _(0y) +n _(z))/3≡n _(LC)′  (3)

wherein the reference symbol n_(z) represents a refractive index in thedirection of the longer axis of the optical indicatrix shown in FIG. 4.

When the equation (2) mentioned above is applicable and n_(z) isrepresented as a refractive index n_(e) of an extraordinary ray, a meanrefractive index n_(LC) is given by the following formula (4):

(2n ₀ +n _(e))/3n _(LC)  (4)

In this case, Maxwell-Garnett's law gives a refractive index n_(A) ofthe polymer dispersive liquid crystal layer 14 by the following equation(5):

n _(A) =ff·n _(LC)′+(1−ff)n _(P)  (5)

wherein the reference symbol n_(P) represents a refractive index of thepolymers which compose the polymer cells 18 and the reference symbol ffdesignates ratio of a volume of the liquid crystal molecules 17 to avolume of the polymer dispersive liquid crystal layer 14.

Accordingly, a focal length f₁ of the vari-focal lens component 11 isgiven by the following equation (6):

1/f ₁=(n _(A)−1)(1/R ₁−1/R ₂)  (6)

wherein the reference symbols R₁ and R₂ represent radii of curvature oninside surfaces of the lens elements 12 a and 12 b respectively, i.e.,on surfaces thereof which are located on a side of the polymerdispersive liquid crystal layer 14. R₁ and R₂ are taken as positive whena center of curvature is located on a side of an imaging point. Further,refraction by outside surfaces of the lens elements 12 a and 12 b arenot of consideration. That is, a focal length of the vari-focal lenscomponent which is composed only of the polymer dispersive liquidcrystal layer 14 is given by the equation (6).

When a mean refractive index n₀′ for the ordinary ray is expressed by aformula (7) shown below, a refractive index n_(B) of the polymerdispersive liquid crystal layer 14 in the condition shown in FIG. 6where the electric field is applied to the polymer dispersive liquidcrystal layer 14 is given by the following equation (8):

n ₀′=(n _(0x) +n _(0y))/2  (7)

n _(B) =ff·n ₀′+(1−ff)n _(P)  (8)

In this case, a focal length f₂ of the vari-focal lens component whichis composed only of the polymer dispersive liquid crystal layer 14 isgiven by the following equation (9):

1/f ₂=(n _(B)−1)(R ₁−1/R ₂)  (9)

When a voltage which is lower than that in FIG. 6 is applied to thepolymer dispersive liquid crystal layer 14, the vari-focal lenscomponent 11 has a focal length which is between the focal length f₁given by the equation (6) and the focal length f₂ given by the equation(9).

From the equations (6) and (9) described above, the polymer dispersiveliquid crystal layer 14 varies a focal length at a ratio given by thefollowing equation (10):

|(f ₂ −f ₁)/f ₂|=|(n _(B) −n _(A))/(n _(B)−1)  (10)

This variation ratio |(f₂−f₁)/f₂| can therefore be enhanced byincreasing |n_(B)−n_(A)|, n_(B)−n_(A) is given by the following equation(11):

n _(B) −n _(A) =ff(n ₀ ′−n _(LC)′)  (11)

It is therefore possible to enhance the variation ratio by enlarging|n₀′−n_(LC)′|. Since n_(B) for practical use is on the order of 1.3 to2, it is sufficient that |n₀′−n_(LC)′| has a value within a rangedefined by the following condition (12):

0.01≦|n ₀ ′−n _(LC)′|≦10  (12)

As far as |n₀′−n_(LC)′| has a value within the range defined by thecondition (12), it is possible to vary a focal length at 0.5% or ahigher ratio with the polymer dispersive liquid crystal layer 14,thereby obtaining an effective vari-focal lens component. Due to arestriction imposed on liquid crystal substances, |n₀′−n_(LC)′| cannothave a value exceeding 10.

Now, description will be made of a basis of the upper limit of thecondition (1).

Variations of transmittance τ caused by varying sizes of polymer liquidcrystals are described in “Transmission variation usingscattering/transparent switching films” of “Solar Energy Materials andSolar Cells”, Vol 31, Wilson and Eck. 1993, Eleevier Science PublishersB.v., pp 197-214. Representing a radius of a polymer liquid crystal byr, and assuming t=300 μm, ff=0.5, n_(P)=1.45, n_(LC)=1.585 and λ=500 nm,FIG. 6 on page 206 of this literature shows a fact that transmittance τhas theoretical values of τ≈90% at r=5 nm (D=λ/50, D·t=λ·6 μm (D and λin nm also applying to the following)) and τ≈50% at r=25 nm (D=λ/10).

On an assumption that transmittance τ varies according to an exponentialfunction of t, transmittance τ at t=150 μm is presumed as τ≈71% at r=25nm (D=λ/10, D·t=λ·15 μm). Similarly transmittance τ at t=75 μm ispresumed as τ≈80% at r=25 nm (D=λ/10, D·t=λ·7.5 μm).

On the basis of these results, r is 70% to 80% or higher and avari-focal lens component can sufficiently be put to practical use whenit satisfies a condition (13) shown below. At t=75 μm, for example,sufficient transmittance can be obtained at D≦λ/5 μm:

D·t≦λ·15 μm  (13)

Transmittance of the polymer dispersive liquid crystal layer 14 ishigher as n_(P) has a value which is closer to a value of n_(LC)′. Whenn₀′ and n_(P) have values different from each other, on the other hand,transmittance of the polymer dispersive liquid crystal layer 14 islowered. As a mean value of transmittance in the condition shown in FIG.4 and that in the condition shown in FIG. 6, the polymer dispersiveliquid crystal layer 14 has high transmittance when it satisfies thefollowing equation (14):

n _(P)=(n ₀ ′+n _(LC)′)/2  (14)

Since the vari-focal lens component 11 is used as a lens, it isdesirable that it has transmittance which remain substantially unchangedbetween the condition shown in FIG. 4 and that shown in FIG. 6, and isas high as possible. Though polymer materials which are available forcomposing the polymer cells 18 and materials for the liquid crystalmolecules 17 are limited, it is sufficient for practical use that n_(P)has a value which satisfies the following condition (15):

n ₀ ′≦n _(P) ≦n _(LC)′  (15)

When n_(P) satisfies the condition (15) mentioned above, it issufficient that D·t satisfies, in place of the condition (13), thefollowing condition (16):

D·t≦λ·60 μm  (16)

This is because reflectance is proportional to a square of a differencebetween refractive indices of media on both sides of a reflectingsurface according to Fresnel's reflection law, whereby reflection on aborders between the polymers composing the polymer cells 18 and liquidcrystal molecules 17, or lowering of transmittance of the polymerdispersive liquid crystal layer 14, is nearly proportional to a squareof a difference between refractive indices of the polymers and theliquid crystal molecules 17.

Though the foregoing description has been made of the case wheren₀′≈1.45 and n_(LC)′≈1.585, it is generally sufficient that D·tsatisfies the following condition (17):

D·t≦λ≦15 μm·(1.585−1.45)²/(n _(u) −n _(P))²  (17)

wherein (n_(u)−n_(P))² is (n_(LC)′−n_(P))² or (n₀′−n_(P))² whichever islarger.

Though a larger value of ff is more advantageous for a large variationof a focal length of the vari-focal lens component 11, ff=1 zeroes avolume of the polymers, thereby making it impossible to form the polymercells 18. Therefore, it is sufficient that ff has a value whichsatisfies the following condition (18):

 0.1≦ff≦0.999  (18)

Further, in order to obtain a higher effect of the vari-focal lenselements, i.e., to make a larger variation of a focal length, it isdesirable that ff has a larger value, or a value not smaller than 0.5 soas to satisfy the following condition (18-5):

0.5≦ff≦0.999  (18-5)

Since τ is enhanced as ff has a smaller value, on the other hand, it isdesirable that D·t satisfies, in place of the condition (17), thefollowing condition (19):

4×10⁻⁶ [μm]² ≦D·t≦λ·45 μm·(1.585−1.45)²/(n _(u) −n _(P))²  (19)

Further, a lower limit of t lies at D as apparent from FIG. 1 and alower limit of D·t lies at (2×10⁻³ μm)², or 4×10⁻⁶ [μm]², since D is notshorter than 2 nm as described above.

The foregoing description is made on an assumption that favorable valuesare demanded for light scattering by the vari-focal lens element andtransmittance thereof. However, optical system, image pickup apparatus,illumination system, signal processing systems, etc. which are to bemanufactured at low costs may not require so favorable scattering andtransmittance and it is sufficient in such cases to satisfy, in place ofthe condition (19), the following condition (19-5)

 4×10⁻⁶ [μm]² ≦D·t≦λ·450 μm (1.585−1.45)²/(n _(u) −n _(P))²  (19-5)

Furthermore, approximations of optical characteristics of substances toexpressions of refractive indices are valid only in cases where D islarger than 10 nm to 5 nm as described in “Minor Planets will Come inIwanami Science Library 8” Tadashi Mukai, 1994, P 58. When D exceeds500λ, rays are scattered geometrically and scattering of rays on theinterfaces between the polymers composing the polymer cells 18 and theliquid crystal molecules 17 is increases according to Fresnel'sreflection formula. It is therefore sufficient for practical use that Dis within a range defined by the following condition (20):

7 nm≦D≦500λ  (20)

In the composition shown in FIG. 4 or FIG. 7, n_(0x), n_(0y), n₀, n_(z),n_(e), n_(P), ff, D, t, λ, R₁, R₂, n_(LC)′, n_(LC), n_(A), n_(B), f₁, f₂and a diameter φ of the vari-focal lens component 11 have, as anembodiment, values which are listed below:

n_(0x)=n_(0y)=n₀=1.5

n_(Z)=n_(e)=1.75

n_(P)=1.54

ff=0.5

D=50 nm

t=125 μm

λ=500 nm

R₁=25 mm

R₂=∞

n_(LC)′=n_(LC)=1.5833

n_(A)=1.5617

n_(B)=1.52

f₁=44.5 mm

f₂=48.04 mm

φ=5 mm

In this case, the right side of the above-mentioned formula (19) is:

45 μm·(1.585−1.45)²·(n _(u) −n _(P))²=500 nm·45μm·(0.135)²/(0.0433)²≈218712 nm·μm

Further, D·t is:

D·t=50 nm·125 μm=6250 nm

Hence, the formula (19) is surely satisfied.

In the embodiment described above, both R₁ and R₂ may be infinite. Insuch a case, an optical path length of the polymer dispersive crystallayer 14 is changed by turning on and off a voltage, whereby thevari-focal lens component 11 may be disposed at a location of a lenssystem where a light bundle is not parallel and used for adjusting afocused condition or changing a focal length of the lens system as awhole.

FIG. 8 shows a composition of an image pickup optical system for digitalcameras which uses the vari-focal lens component 11 shown in FIG. 7.This image pickup optical system forms an image of an object (not shown)on a solid-state image pickup device 23 which is composed, for example,of a CCD by way of a stop 21, the vari-focal lens component 11 and alens component 22. In FIG. 8, liquid crystal molecules are not shown.

When a focal length of the vari-focal lens component 11 is changed byadjusting an AC voltage applied to a polymer dispersive liquid crystallayer 14 of the vari-focal lens component 11 with a variable resistor19, it is possible to focus this image pickup optical systemcontinuously on object distances from infinite to 600 mm, for example,without moving the vari-focal lens component 11 and the lens component22 in a direction along an optical axis.

FIG. 9 shows a composition of an objective optical system for electronicendoscopes which uses the vari-focal lens component according to thepresent invention. This objective optical system forms an image of anobject (not shown) on a solid-state image pickup device 29 which iscomposed, for example, of a CCD by way of a front lens component 25, astop 26, a vari-focal lens 27 and a rear lens component 28. Thevari-focal lens component 27 has a composition which is the same as thatshown in FIG. 7, except for an inside surface of a lens element 12 adisposed on one side of a polymer dispersive liquid crystal layer 14which is configured as a planar surface having an infinite radius ofcurvature R₁ and an inside surface of another lens element 12 b which isconfigured as a Fresnel lens surface so that an AC voltage is applied tothe polymer dispersive liquid crystal layer 14 from an AC power source16 by way of a variable resistor 19 and a switch 15. Liquid crystalmolecules are not shown in FIG. 9.

By adjusting an AC voltage applied to the polymer dispersive liquidcrystal layer 14 dependently on object distances to change a focallength of the vari-focal lens component 27, it is also possible toperform focus adjustments of this objective optical system withoutmoving the vari-focal lens component 27 and the rear lens component 28along an optical axis.

FIG. 10 exemplifies a composition of a vari-focal diffractive opticalelement according to the present invention (the vari-focal lenscomponent using a diffractive optical element according to the presentinvention). A vari-focal diffractive optical element 31 comprises afirst transparent substrate (first optical member) 32 having first andsecond surfaces 32 a and 32 b in parallel with each other, and a secondtransparent substrate (second optical member) 33 having a third surface33a forming a ring-like diffraction grating which has a sawtooth-shapedsection having a groove depth on the order of a wavelength of a ray anda fourth planar surface 33 b: the vari-focal diffractive optical elementbeing configured so as to allow rays to emerge through the first andsecond transparent substrates 32 and 33. A polymer dispersive liquidcrystal layer 14 is disposed between the first and second transparentsubstrates 32 and 33 by way of the transparent electrodes 13 a and 13 bas in the composition described with reference to FIG. 1, and thetransparent electrodes 13 a and 13 b are connected to an AC power source16 by way of a switch 15 so that an AC electric field is applied to thepolymer dispersive liquid crystal layer 14.

Applicable to the composition described above is the following formula(21):

p sin θ=mλ  (21)

wherein the reference symbol p represents a pitch of gratings on thethird surface 33 a for rays incident on the vari-focal diffractiveoptical element 31 (the optical member having the diffraction grating)and the reference symbol m designates an integer.

That is, the incident rays emerge at an angle of deflection θ. When thefollowing equations (22) and (23) are satisfied, a diffractionefficiency is 100% at a wavelength λ, thereby allowing to prevent flarefrom being produced:

h(n _(A) −n ₃₃)=mλ  (22)

h(n _(B) −n ₃₃)=kλ  (23)

wherein the reference symbol h represents a groove depth, the referencesymbol n₃₃ designates a refractive index of the transparent substrate 33and the reference symbol k denotes an integer.

By subtracting both the sides of the equation (23) from both the sidesof the equation (22), we obtain the following equation (24):

h(n _(A) −n _(B))=(m−k)λ  (24)

Assuming that λ=500 nm, n_(A)=1.55 and n_(B)=1.5, for example, theequation (24) is:

0.05h=(m−k)·500 nm

When m=1 and k=0, h is calculated as follows:

h=10000 nm=10 μm

As judged from the equation (22) mentioned above, it is sufficient inthis case that the transparent substrate 33 has a refractive indexn₃₃=1.5. When the grating has a pitch P of 10 μm at a marginal portionof the vari-focal diffractive optical element 31, θ≈2.87°, whereby alens component which has an F number of 10 can be obtained.

Since the vari-focal diffractive optical element 31, thus obtained hasan optical path length which is changed by turning on and off a voltageapplied to the polymer dispersive liquid crystal layer 14 and, it can bedisposed at a location of a lens system where a light bundle is notparallel and used for adjusting a focused condition, changing a focallength of a lens system as a whole or another purpose.

For practical use, it is sufficient that the embodiment satisfies, inplace of the equations (22) through (24), the following conditions (25),(26) and (27):

0.7mλ≦h(n _(A) −n ₃₃)≦1.4mλ  (25)

0.7kλ≦h(n _(B) −n ₃₃)≦1.4kλ  (26)

0.7(m−k)λ≦h(n _(A) −n _(B))≦1.4(m−k)λ  (27)

FIGS. 11 and 12 show vari-focal spectacles (spectacles using vari-focallens components) 35 which use a vari-focal diffractive optical element36 as a spectacle lens component. The vari-focal diffractive opticalelement 36 has lens elements 37 and 38, and a ring-like diffractiongrating which has a saw-tooth shaped section similar to that describedwith reference to FIG. 10 is formed on an inside surface of the lenselement 37 disposed on the side of incidence. Orientation films 39 a and39 b are disposed on the inside surfaces of the lens elements 37 and 38by way of transparent electrodes 13 a and 13 b respectively, and apolymer dispersive liquid crystal layer 14 similar to that describedwith reference to FIG. 4 is disposed between the orientation films 39 aand 39 b. Further, the transparent electrodes 13 a and 13 b areconnected to an AC power source 16 by way of a switch 15 so that an ACelectric field is applied to the polymer dispersive liquid crystal layer14.

Since orientation of liquid crystal molecules 17 in the polymerdispersive liquid crystal layer 14 is changed between a condition wherethe switch 15 is turned off as shown in FIG. 11 and another conditionwhere the switch 15 is turned on as shown in FIG. 12, the vari-focalspectacle 35 which has the configuration described above is capable ofchanging a diopter of the spectacle as a whole. Accordingly, thevari-focal spectacle 35 according to the present invention shown inFIGS. 11 and 12 does not change a diopter dependently on directions ofan eye, thereby eliminating a feeling of incompatibility unlikeconventional spectacles 42 which use lens components 41 having dualfocal points shown in FIG. 13.

Vari-focal spectacles shown in FIG. 14 is a vari-focal spectacles 35shown in FIG. 11 which is equipped with a range finder sensor 46 formeasuring a distance to the object 45 disposed, for example, on a frame35 a and configured to automatically adjust diopter of the spectacles byperforming on/off control of the switch 15 on the basis of an outputfrom the range finding sensor 46.

By configuring spectacles so as to automatically adjust a diopter on thebasis of object distances as described above, it is possible to obtainspectacles which are convenient for the aged who have weakened diopteradjusting abilities.

Though the spectacle lens component is composed entirely of thevari-focal diffractive optical element 36 in the vari-focal spectacles35 shown in FIGS. 11 and 14, it is possible to dispose the vari-focaldiffractive optical element 36 as a portion of a spectacle lenscomponent, for example, at a location which is a little lower than acenter as shown in FIG. 15. Further, the vari-focal lens component 11shown in FIG. 4 or the vari-focal lens component 27 shown in FIG. 9 maybe used in place of the vari-focal diffractive optical element 36.Though the vari-focal spectacles shown in FIG. 14 is configured to turnover the switch 15 on the basis of the output from the range findersensor 46, it is possible to dispose an additional switch so as topermit selection between the automatic switching with the range findersensor 46 and a manual switching or modification to the manual switchingduring the automatic switching with the range finder sensor 46.Furthermore, it is possible to integrate a hearing aid with thevari-focal spectacles 35 described above.

When the range finder sensor 46 is to be disposed on the vari-focalspectacles as shown in FIG. 14, it is possible to vary stepwise orcontinuously a voltage to be applied to the polymer dispersive liquidcrystal layer 14 of the vari-focal diffractive optical element 36, andpreset correspondence between an output from the range finder sensor 46and an applied voltage dependently on a user so as to control theapplied voltage on the basis of the output from the range finder sensor46. By controlling the applied voltage as described above, it ispossible to adjust a diopter more accurately and automatically for eachuser dependently on object distances.

The AC power source 16 for the vari-focal spectacles 35 described abovecan be composed of an inverter circuit which uses batteries as its powersource. In this case, the vari-focal spectacles 35 can be equipped withone kind or plural kinds of batteries such as manganese batteries,lithium batteries, solar batteries and rechargeable batteries, which maybe integrated with the frame 35 a or built therein, separately disposedand connected by way of cords or consists of a built-in battery and anexternal battery.

For simply composing vari-focal spectacles, it is possible to adoptvari-focal lens components which use a twisted nematic liquid crystal ora liquid crystal having a twisted orientation such as a chorestericliquid crystal, in place of the vari-focal lens components which use thepolymer dispersive liquid crystal described above. FIGS. 16 and 17 showa configuration of vari-focal spectacles 50 using a twisted nematicliquid crystal, wherein a vari-focal lens component 51 is composed oflens elements 52 and 53, orientation films 39 a and 39 b which aredisposed on inside surfaces of these lens elements by way of transparentelectrodes 13 a and 13 b, and a twisted nematic liquid crystal layer 54which is disposed between the orientation films: the transparentelectrodes 13 a and 13 b being connected to an AC power source 16 by wayof a variable resistor 19 so that an AC electric field is applied to thetwisted nematic liquid crystal layer 54.

When a voltage applied to the twisted nematic liquid crystal layer 54 isenhanced in the vari-focal spectacles which has the configurationdescribed above, liquid crystal molecules 55 are homeotropicallyoriented as shown in FIG. 17, whereby the twisted nematic liquid crystallayer 54 has a lower refractive index and a longer focal length ascompared with the twisted nematic condition shown in FIG. 16 where alower voltage is applied.

Since a spiral pitch P of the liquid crystal molecules 55 must besufficiently short as compared with a wavelength λ of rays in thetwisted nematic condition shown in FIG. 16, it is desirable to satisfy,for example, the following condition (28):

2 nm≦P≦2λ/3  (28)

The lower limit (2 nm) of the condition (28) is determined by a size ofliquid crystal molecules and the upper limit (2λ/3) is required to allowthe twisted nematic liquid crystal layer 54 to behave as an isotropicmedium in the condition shown in FIG. 16 when natural light is incident.If the pitch P has a value exceeding the upper limit, the vari-focallens component 51 has a focal length which is different dependently ondirections of polarization, thereby forming a dualized or blurred image.

However, high optical performance may not be demanded for practical usein certain cases, and it is sufficient in such case to satisfy, in placeof the condition (28), the following condition (28-5):

2 nm≦P≦40λ  (28-5)

A form and a design of a frame 35 a of spectacles are usually selectedas desired by a user.

For allowing a user to optionally select a frame, it is convenient toconfigure a component which consists of a power source 16, a switch 15,cords 150, etc. for vari-focal lens components to be used withspectacles 35 shown in FIG. 18, for example, as a separate component andfix it to the spectacles 35 after electrically connecting these parts tothe vari-focal spectacles.

An example of the spectacles described above is shown in FIG. 18,wherein a reference numeral 151 represents spectacle lens componentswhich use a liquid crystal, a reference numeral 152 designates a drivingunit: the spectacle lens components being electrically connected to thedriving unit by way of cords 150. The vari-focal spectacle lenscomponents 151 which use the liquid crystal and the driving unit 152 maybe manufactured separately and coupled with one another.

FIG. 19 shows a condition where the vari-focal spectacle lens components151 using the liquid crystal and the driving unit 152 are attached to aspectacle frame 154, and the cords 150 are fixed with fixing means 153such as bands or heat-shrinkable rings or adhesive tape. The frame 154may be used as a user likes. The driving unit 152 may be put in a pocketor the like. Alternately, the driving unit 152 can be put on a head likea headphone as shown in FIG. 20 or hung behind ears, under the occipitalregion or on the neck as shown in FIG. 21. In this case, the cords maybe disposed in or on sidepieces of the frame 154 so that the cords 150can be adopted by replacing only the sidepieces of the frame 154. Forexample, it is conceivable to pass the cords through slots formed in thesidepieces of the frame 154 or form cords by printed wiring as shown inFIG. 19. A reference numeral 155 represents an AC current generatingcircuit, for example, an oscillator circuit or an inverter circuit.

FIG. 22 shows an example wherein a switch 156 is disposed at a locationof an outside surface of a lens component, thereby making it possible tochange a focal length simply by touching the outside surface of the lenscomponent, or without touching a driving unit 152 unlike the examplewherein the switch is disposed on the driving unit 152. When the switch156 is configured as a touch-switch, it can be conveniently manipulatedwith a weak force or a light touch.

FIG. 23 shows an example wherein a circuit for driving a vari-focal lenscomponent other than a power supply is formed at an outercircumferential portion by using a transistor manufacturing technique orthe like. This circuit permits composing a driving unit 157 so as tohave a simple configuration and a light weight, thereby providing auser's convenience.

FIG. 24 shows an example of lens component which can be combined withvarious frames, wherein rather a large vari-focal lens component 158 isformed so as to permit cutting out a portion 158 a thereof which ismatched with a frame. In this example, a vari-focal lens portion isformed as a section indicated by a reference numeral 159 which is formedinside the member 158.

When a vari-focal lens component is formed in a shape described above,it can be shaped so as to match with various frames.

For security against power failure during driving of automobiles, forexample, all of the vari-focal spectacles described above are to beconfigured so that they are focused on long object distances in caseswhere the power sources are turned off due to complete discharge ofbatteries or wire breakage or cases where the driving unit 152 becomesdefective. Such a configuration is effective to lower a powerconsumption when a user mainly gazes into the distance for a long time.

For this purpose, a polymer dispersive liquid crystal layer isconfigured so as to have a function of a concave lens as shown in FIG.25 so that it exhibits a function of a concave lens which is stronger ina power-off condition than that in a power-on condition, whereby thespectacles are focused on long object distances.

When a user mainly gazes at objects, etc. located at short distances, incontrast, it is advantageous for preventing power sources such asbatteries from being consumed to configure the vari-focal spectacles tobe focused on short distances in conditions where the power sources areturned off or voltages are low.

On the other hand, certain users of spectacles mostly gaze at images athigh contrast of objects which are located at long distances and look atobjects located at short distances only for a short time. It isdesirable for these users to turn on the power sources so that thespectacles are focused on objects located at long distances at highvoltages. When voltages are high as described above, liquid crystalmolecules fluctuate little, thereby making it possible to obtain imagesof high contrast.

For users of spectacles who gaze at images at high contrast of objectlocated at short distances for a long time, in contrast, it is desirableto configure spectacles so that they are focused on short objectdistances when power sources are turned on or voltages are set at highlevels.

That is to say, it is desirable that the power sources are turned on orthe voltages are set at the high levels when the users of the spectacleswant to see images with high contrast.

Since various persons such as short-sighted persons, far-sighted personsand astigmatic persons use spectacles, it is necessary to configurespectacles so as to be matched with each of the persons. Therefore, itis advantageous to compose one of the two substrates required forcomposing vari-focal spectacle lens as a common part and configure theother substrate selectively as a convex lens, a concave lens or acylindrical lens for an astigmatic eye dependently on a user so that oneof the substrates can be used commonly, thereby lowering a manufacturingcost.

Since a liquid crystal has an Abbe's number which is smaller than thatof a glass material, a liquid crystal lens produces remarkable chromaticaberration. For correcting this chromatic aberration, it is preferableto combine a liquid crystal lens which has a function of a convex lenswith a substrate (optical member) which has a function of a concave lensor combine a liquid crystal lens which has a function of a concave lenswith a substrate (optical member) which has a function of a convex lens.

FIG. 25 shows an example of such a combination type vari-focal spectaclelens component which consists of a Fresnel lens element 160 of a polymerdispersive liquid crystal which has a function of a convex lens elementand a substrate 161 which has a function of a concave lens.

In case of a vari-focal spectacle lens component which uses adiffractive optical element, it produces chromatic aberration in adirection reverse to that of chromatic aberration produced by thespectacle lens component described above. It is therefore preferable tocombine a diffractive optical element which has a function of a convexlens with a substrate which has a function of a convex lens or combine adiffractive optical element which has a function of a concave lens witha substrate which has a function of a concave lens.

The vari-focal spectacle lens component shown in FIG. 25 is configuredto apply a bias voltage with a resistor 162 when the switch 15 is turnedon so as to enhance a response to a change of a focal length by varyinga voltage with a variable resistor 19.

For preventing breakage of the liquid crystal in this liquid crystallens component, it is preferable to select a material which does notcontain sodium for the substrate.

The vari-focal spectacle lens components described above require twocords for connection to a driving unit. It is desirable to connect thesetwo cords to the driving unit 152 collectively from one of the spectaclelens components as shown in FIG. 26. When the cords are arranged asshown in FIG. 26, the spectacles 35 can be used conveniently sincesubstantially a single cord is connected to the spectacles 35 and cannothitch while the spectacles 35 is being put on and off. Cords 150 whichare collected as described above may be led out of a sidepiece 154 asshown in FIG. 26 or the vicinity of one of the lens components.

It is important for practical use to arrange the cords coming out of thespectacles 35 not in two systems but in a single system. It ispreferable to allow the single system of cords to come out on a sideopposite to the skilful hand of a user so that the cords will notconstitute a hindrance to the user.

FIG. 27A shows a composition of the variable declination prism accordingto the present invention. This variable declination prism 61 has a firstincidence side transparent substrate (first optical member) 62 which hasfirst and second surfaces 62 a and 63 b, and a second emergence sidetransparent substrate (second optical member) 63 which has third andfourth surfaces 63 a and 63 b, and a shape of a plane parallel plate. Aninside surface (the second surface) 62 b of the incidence sidetransparent substrate 62 is configured in a Fresnel shape, and a polymerdispersive liquid crystal layer 14 is disposed between the transparentsubstrate 62 and the emergence side transparent substrate 63 by way oftransparent electrodes 13 a and 13 b similarly to the polymer dispersiveliquid crystal layer which has been described with reference to FIG. 4.The transparent electrodes 13a and 13 b are connected to an AC powersource 16 by way of a variable resistor 19 so that declinations of raystransmitting through the variable declination prism 61 are controlled byapplying an AC electric field to the polymer dispersive liquid crystallayer 14. Though the inside surface 62 b of the transparent substrate 62is configured in the Fresnel shape in FIG. 27A, it is possible, forexample, to configure the variable declination prism so as to have anordinary form of a prism wherein inside surfaces of the transparentsubstrates 62 and 63 are inclined toward each other as shown in FIG. 27Bor a comprise a surface of a diffraction grating as shown in FIG. 10.When a surface is configured as a diffraction grating, the equations(21) through (27) are applicable.

The variable declination prism 61 which has the configuration describedabove is usable for effectively preventing vibrations, for example, ofTV cameras, digital cameras, film cameras and binoculars. It isdesirable to configure the variable declination prism 61 so as torefract (deflect) rays in a vertical direction and it is more desirablefor obtaining improved performance to dispose two variable declinationprisms 61 so as to vary angles of refraction in two differentdirections, for example, in the vertical direction and a horizontaldirection which are perpendicular to each other as shown in FIG. 28.Liquid crystal molecules are not shown in FIGS. 27 and 28.

FIG. 29 shows a vari-focal mirror which is configured as the vari-focallens component according to the present invention. This vari-focalmirror 65 comprises a first transparent substrate 66 which has first andsecond surfaces 66 a and 66 b, and a second transparent substrate 67which has third and fourth surfaces 67 a and 67 b. The first transparentsubstrate 66 is configured so as to have a form of a planar plate or alens and a transparent electrode 13 a which is disposed on its insidesurface (the second surface) 66 b. The second transparent substrate 67has an inside surface (the third surface) 67 a, which is configured as aconcave surface, a reflective film 68 which is formed on this concavesurface and a transparent electrode 13 b which is disposed on thereflective film 68. A polymer dispersive liquid crystal layer 14 isdisposed between the transparent electrodes 13 a and 13 b similarly tothe polymer dispersive liquid crystal layer 14 which has been describedwith reference to FIG. 4. These transparent electrodes 13 a and 13 b areconnected to an AC power source 16 by way of a switch 15 and a variableresistor 19 so that an AC electric field is applied to the polymerdispersive liquid crystal layer 14. Liquid crystal molecules are notshown in FIG. 29.

Since the configuration described above forms an optical path whichallows the reflective film 68 to reflect rays incident on thetransparent substrate 66 so as to return the rays through the polymerdispersive liquid crystal layer 14, the vari-focal mirror 65 allows thepolymer dispersive liquid crystal layer 14 to function twice and permitschanging a focused point of the reflected rays by changing a voltageapplied to the polymer dispersive liquid crystal layer 14. Since rayswhich are incident on the vari-focal mirror 65 transmit through thepolymer dispersive liquid crystal layer 14 twice, the equationsmentioned above are similarly applicable when twice a thickness of thepolymer dispersive liquid crystal layer 14 is taken as t. The thicknessof the polymer dispersive liquid crystal layer 14 can be reduced byconfiguring the inside surface of the transparent substrate 66 or 67 soas to have a form of a diffraction grating as shown in FIG. 10. Such aconfiguration will provide a merit to reduce scattered rays. Inaddition, it is possible configure the reflective film 68 so as to havea function of electrode without using the transparent electrode 13 b.

Though the AC power source 16 is used for applying an AC electric fieldto the liquid crystal for preventing deterioration of the liquid crystalin the embodiments described above, it is possible to apply a DCelectric field with a DC power source. Further, directions of liquidcrystal molecules can be changed by varying not only a voltage but alsoa frequency of an electric field applied to a liquid crystal, anintensity or a frequency of a magnetic field applied to a liquid crystalor a temperature of a liquid crystal.

Polymer dispersive liquid crystals are available not only in liquidstates but also in nearly solid states.

When a polymer dispersive liquid crystal which is in a nearly solidstate is to be used as the polymer dispersive liquid crystal layer inthe embodiments described above, it is possible to omit at least one ofthe first and second optical members, for example, either one of thelens elements 12 a and 12 b shown in FIG. 4, the transparent substrate32 shown in FIG. 10, the lens element 38 shown in FIG. 11, at least oneof the lens elements 52 and 53 shown in FIG. 16, either one of thetransparent substrates 62 and 63 shown in FIGS. 17A and 17B or at leastone of the transparent substrates 66 and 67 shown in FIG. 29.

Now, description will be made of a manufacturing method of aheterogeneous medium lens element which is used as one of the lenselements.

A heterogeneous medium lens element is a lens element having arefractive index of medium which is different from portion to portionthereof. A heterogeneous medium lens element 71 which is configured as aradial type gradient index lens element having a refractive indexvarying in a radial direction as shown in FIG. 30, for example, has arefractive index n which is lowered as a radius r is longer from anoptical axis 0 located at an axis (a center axis of a refractive indexdistribution) as shown in FIG. 31. The refractive index remainsunchanged in a direction along the optical axis.

A material of a heterogeneous medium lens element such as the lenselement 71 is manufactured from a material such as a glass or plasticmaterial by the ion exchange method, sol-gel method or the like.However, a material 72 of a heterogeneous medium lens elementmanufactured by this method has a rod-like form as shown in FIG. 32, andit is necessary to subject the material to cutting, polishing, coatingand other steps for obtaining a heterogeneous medium lens element 71 asa final product.

On the other hand, an ordinary homogeneous lens element made of a glassmaterial or the like is manufactured by polishing both surfaces of alens element and then cutting its outer circumference so as to berevolutionarily symmetrical with regard to a line passing throughcenters of two spherical surfaces (an optical axis). This method is usedgenerally and widely since it has a merit to permit manufacturing a lenselement at a low cost.

Unlike the working of the ordinary lens element, however, manufacturingof the heterogeneous medium lens element requires that the axis of thematerial 72 is located at a center of an outer circumference and that alens surface 73 is perpendicular to the axis as shown in FIG. 30. Whenthe heterogeneous medium lens element is manufactured by the workingmethod for the ordinary lens element described above, the axis maydeviate or incline from the center of the outer circumference of thelens element and the lens surface 73 may not be accurately perpendicularto the axis.

A manufacturing method of a heterogeneous lens element which is capableof solving such a problem will be described with reference to theaccompanying drawings.

First, description will be made of a manufacturing method of aheterogeneous lens element having a lens surface 73 which is planar.Description will be made on an assumption that the axis is located atthe center of the material 72 since an axis is actually coincident witha center of a material which is manufactured by the ion exchange method,sol-gel method or the like. First, the material 72 is cut with acenterless grinder 75 until it has a small diameter so as not to deviatethe axis from the center of the material 72 as shown in FIG. 33. Adistance between two shafts of the centerless grinder 75 ispreliminarily adjusted so that a material 72 a which is cut thin willhave an outside diameter required for a lens element.

Then, the material 72 a is cut into a material 72 b which has a requiredlength including polishing margins with a cutter 76 as shown in FIG. 34.The cut material 72 b is thereafter bonded to a side surface of aV-shaped groove of a V block 77 mounted on a flat surface as shown inFIG. 35 so that an axis is perpendicular to the flat surface or to aside surface of a tooth groove of a gear 78 as shown in FIG. 36 so thatthe axis of the heterogeneous lens element is in parallel with a centeraxis of the gear 78.

Then, the material 72 b which is bonded to the V block 77 or the gear 78is set on a surface grinder 81 as shown in FIG. 37 and one lens surfaceof the material 72 b is cut with a diamond grind stone so that it isperpendicular to the axis. The lens surface is thereafter cut preciselyat several steps with a polishing machine 82 using progressively finerdiamond pellets 82 a as shown in FIG. 38, and finished into a mirrorsurface by polishing with CeO₂ and water using a urethane sheet, pitchor the like. The other lens surface is also finished into a mirrorsurface by working it as shown in FIGS. 35 through 38.

After both the lens surfaces are chamfered with an engine lathe 84 orthe like as shown in FIG. 39, each of the surfaces is coated with amono-layer or multi-layer reflection preventive of MgF₂ or the similarsubstance, thereby obtaining a heterogeneous medium lens element whichhas two planar surfaces.

When perpendicularity of a cut surface (the lens surface) to the centeraxis of a refractive index distribution is maintained at the cuttingstep shown in FIG. 34, it is possible to perform the precise cutting andpolishing of the lens surface shown in FIG. 37 and FIG. 38 withoutbonding the heterogeneous lens element to the V block 77 or the gear 78as shown in FIG. 35 or 36. After completing polishing of one of the lenssurfaces at the step shown in FIG. 38, the polished surface may bebutted and bonded to a bonding dish 86 as shown in FIG. 40, and theother lens surface may be precisely cut and polished with the polishingmachine 82 in the condition shown in FIG. 40. This practice simplifiesthe manufacturing steps, thereby providing an advantage from a viewpointof a manufacturing cost.

Another manufacturing method of a heterogeneous medium lens elementwhich has a planar lens surface 73 is to perform the cutting step shownin FIG. 34 without carrying out the outside diameter cutting step shownin FIG. 33, and carry out the steps shown in FIGS. 35 through 38 forboth lens surfaces. After completing polishing of the lens surfaces, amaterial 72 c is bonded to a sider type centering machine 90 by way ofpitch 88 as shown in FIG. 41. The material 72 c is bonded to the sidertype centering machine 90 while checking with a pick tester 91 orobserving an outer circumference of the material 72 c through amicroscope 92 so that the material 72 c will not be vibrated when thesider type centering machine is rotated. While rotating the sider typecentering machine 90 in this condition, the outer circumference of thematerial 72 c is cut with a grind stone 93 until it has an outsidediameter of a finished lens element. Then, the surfaces are chamferedwith the sider type centering machine 90 and coated with anantireflection film, thereby obtaining a heterogeneous medium lenselement which has a center coincident with an axis.

Though description has been made above of the manufacturing methods of aheterogeneous medium lens element which has two planar surfaces, thesemethod are effectively applicable to manufacturing of a heterogeneousmedium lens element which has a planar surface and working foruniformalization of outside diameters after lens elements are finished.

Then, description will be made of an example of manufacturing method ofa heterogeneous medium lens element which has spherical surfaces havinga radius R. First, a lens surface member 95 which has a surface having aradius R is prepared from a glass, metal, resin or other material asshown in FIG. 42. Then, this lens surface member 95 is bonded to arotating shaft of a sider type centering machine so as to be free fromeccentricity. While rotating the rotating shaft, a hole into which a cutheterogeneous medium lens element is to be fitted is formed at a centerof the lens surface member 95 with a diamond grind stone 96 and an outercircumference of the lens surface member 95 is chamfered. The lenssurface member in which the hole is formed will hereinafter be referredto as a fixture 97.

A cut heterogeneous medium lens material 72d is fitted into the hole ofthe fixture 97 and fixed with plaster or the like as shown in FIG. 43.In this condition, a spherical surface which is not eccentric from anaxis of a heterogeneous medium lens element is formed by preciselycutting and polishing the material 72 d and the fixture 97 with apolishing machine 82. A heterogeneous medium lens material 72 c which isto be fitted into the hole of the fixture 97 can preliminarily be cutwith a centerless grinder so that it has a diameter of a finished lenselement. Needless to say, the hole of the fixture 97 has in this case aninside diameter which is nearly equal to outer diameter of a finishedlens element.

The other surface may be formed into a spherical surface in the similarin the similar procedure. When a lens element is to have a thin marginalportion (a size of an outer circumferential surface in a direction alongan optical axis) in particular, the other surface can be formed into aspherical surface in the following procedures. A heterogeneous mediumlens material 72 e which has a surface polished into a spherical surfaceis bonded, on a side of the spherical surface, to a fixture 101 whichhas a concave surface having a radius R by way of a pitch 88 as shown inFIG. 45. While checking with a pick tester 91 or observing an outercircumference of the material 72 e through a microscope 92 as describedwith reference to FIG. 41, the material 72 e is bonded so that it willnot be vibrated when the fixture 101 is rotated. The fixture 101 has adiameter which is nearly equal to that of a rotating shaft of a curvegenerator described later or a centering machine so that it can beattached to the curve generator or the centering machine in thecondition where the material 72 e is bonded thereto. For facilitatingthe bonding work, it is preferable to select, out of two sphericalsurfaces to be finally formed, one whichever has a longer radius ofcurvature as a surface to be bonded to the fixture 101.

The fixture 101 is attached to a curve generator 102 as shown in FIG. 46and the other surface of the material 72 e (a right side surface in thedrawing) is cut so as to have desired curvature. Then, the fixture 101is attached to a centering machine and an outer circumference of thematerial 72 e is cut until it has an outside diameter equal to that of afinished lens element. After polishing the other surface of the material72 e into a mirror surface with a polishing machine, both the surfacesare coated as required, thereby obtaining a heterogeneous lens elementhaving two spherical surfaces.

The step to cut the outer circumference of the material 72 e with thecentering machine may be carried out after the other surface ispolished. In this case, the outer circumference of the material 72 e maybe cut using a bell clamp centering machine 103 as shown in FIG. 47. Forpolishing the outer circumference of the material 72 e after polishingthe other surface, the material 72 e which has the two polished surfacesmay be bonded to the fixture 101 by way of pitch 88 as shown in FIG. 48and cut with a grind stone while rotating the fixture 101. In this case,the material 72 e is bonded to the fixture 101 while observing through amicroscope 92 so that it will not be vibrated when the fixture isrotated. Alternately, the outer circumference of the material 72 e maybe cut without using the microscope 92 but while observing vibrations ofa reflected image of the spherical surface (the right side surface inFIG. 48) with an ordinary sider type centering machine.

A heterogeneous medium lens element having spherical surfaces, one whichhas a thick marginal portion, i.e., an outer circumferential surfacehaving a large size in a direction along an optical axis in particular,can be manufactured not only by the methods described above but also amethod described below. First, a cut material 72 b is fitted into acollect chuck 105 as shown in FIG. 49. After one surface is cut into adesired spherical surface with a curve generator 102, it is preciselycut and then polished with pitch into a mirror surface. Then, thematerial 72 b is fitted into another collect chuck 107 which has a pipe106 for supporting a spherical surface so that the other surface (asurface which is not polished) of the material 72 b is set outside asshown in FIG. 50 and the other surface is cut into a desired sphericalsurface with the curve generator 102. The pipe 106 is used for obtaininga desired thickness L of a lens element and preventing the sphericalsurface from being eccentric from an axis of the material 72 b. Then,the other surface which is cut into the spherical surface is preciselycut and polished with pitch into a mirror surface. An outercircumference of the material 72 b is cut as occasion demands until ithas an outside diameter of a finished lens element by any one of themethods described, and then the surfaces are chamfered and coated,thereby obtaining a heterogeneous medium lens element having twospherical surfaces. Though a heterogeneous medium lens element ismanufactured by the method described above, a heterogeneous medium lenselement which has aspherical surfaces can be manufactured in the similarprocedures, according to the present invention.

Now, description will be made of embodiments of the image pickup unitaccording to the present invention.

A first embodiment of the image pickup unit according to the presentinvention is a plate-like image pickup unit 207 which is manufactured byforming, as shown in FIG. 51, free curved surfaces 204, 206 and adiffractive optical element (hereinafter referred to as DOE) 205 asoptical elements on both surfaces of a transparent substrate 203 made ofa glass, crystal, plastic or another material, and further forming asolid-state image pickup device 201 by using a thin silicon filmtechnique or the like. A free curved surface is a kind of asphericalsurface which is not always axially symmetrical but usable as a surfacehaving a refractive or reflective function. In this embodiment, a ray Recoming from an object (not shown) is refracted by the free curvedsurface 204, deflected and reflected by the offaxis type DOE 205,reflected by the free curved surface 206, and imaged on the solid-stateimage pickup device 201. Since the free curved surfaces 204, 206 and theDOE 205 correct aberrations, an image which is as favorable as oneimaged by an ordinary lens system is incident on the solid-state imagepickup device 201. The free curved surfaces 204 and 206 may be formed bymolding, and the DOE 205 may be formed by molding or lithographysimultaneously with the solid-state image pickup device 201. Thesolid-state image pickup element 201 may be formed directly on thetransparent substrate 203 by lithography. When it is difficult to formthe solid-state image pickup element 201 directly on the transparentsubstrate 203, however, it may be manufactured separately and integratedwith the transparent substrate 203 at a subsequent step. A mirror may bedisposed on the transparent electrode 203.

A second embodiment of the image pickup unit according to the presentinvention is a unit for portable information terminal wherein the imagepickup unit 207 preferred as the first embodiment is formed on thetransparent substrate 203 together with a TFT liquid crystal display208, IC 209 for a peripheral circuit and a microprocessor 210. The imagepickup unit 207 may be formed together with an IC (LSI) which hasfunctions of a memory, telephone and so on. Further, formed on thetransparent substrate 203 is a viewfinder 211 for an electronic imagepickup unit. This viewfinder may be configured as a simple visual fieldframe formed on the transparent substrate 203 or a Galilean telescopetype viewfinder consisting of a concave lens element 212 and a convexlens element 213 which are disposed on both the surfaces of thetransparent substrate 203 as shown in FIG. 53. A view finder may beformed by adding lenses, etc. to the transparent electrode.

A third embodiment of the image pickup unit according to the presentinvention is a plate-like image pickup unit 214 which is configured soas to be capable of adjusting a focal point as shown in FIG. 54. Foradjusting a focal point with the plate-like image pickup unit 214, it isimpossible to mechanically move the DOE 205, the free curved surface 206and so on shown in FIG. 51. Therefore, the plate-like image pickup unit214 preferred as the third embodiment uses an optical element 215 whichhas a variable optical characteristic. FIG. 55 shows an example of theoptical element 215 which has a vari-focal DOE 217 using a polymerdispersive liquid crystal 216. Grooves on the order of wavelengths ofrays are formed in at least one of surfaces of a transparent substrate218 so that liquid crystal molecules 220 are oriented as shown in FIG.56 by applying a voltage to a transparent substrate 219, therebylowering a refractive index of the polymer dispersive liquid crystal216. When a voltage is not applied, on the other hand, the liquidcrystal molecules 220 are directed at random, whereby the refractiveindex of the polymer dispersive liquid crystal 216 is enhanced.Accordingly, the vari-focal DOE 217 is capable of changing a focallength dependently on whether or not a voltage is applied. When a weightratio of the liquid crystal molecules 220 is enhanced until it exceeds acertain level (for example, not lower than 25%) the polymer dispersiveliquid crystal 216 is nearly solid, thereby making it unnecessary todispose a substrate on the right side of the polymer dispersive liquidcrystal 216. Further, a right side surface of the polymer dispersiveliquid crystal 216 and a left side surface of the transparent substrate218 may be configured as curved surfaces 221 so as to have a function ofa lens and a function to correct aberrations. In each of the examplesshown in FIGS. 55 and 57, a right side surface of the transparentsubstrate 218 may be configured not as a DOE surface but as a Fresnelsurface. In this case, the DOE 217 functions as a vari-focal Fresnellens element. Further, the right side surface of the transparentsubstrate 218 may be configured as a curved surface like a surface of anordinary lens element as shown in FIG. 61.

Furthermore, the transparent substrates 203 and 218 may be configured soas to exhibit effects of infrared cut filters.

A fourth embodiment of the image pickup unit according to the presentinvention is a plate-like image pickup unit which uses a reflection typevari-focal Fresnel mirror 222 as shown in FIG. 58. The Fresnel mirror222 functions as a vari-focal Fresnel mirror since a reflecting surface223 is disposed as shown in FIG. 59 and a refractive power of a Fresnelsurface 226 is changed when a voltage is varied by turning on/off aswitch 224 or with a variable resistor 225. A DOE may be used in placeof the Fresnel surface 226.

A vari-focal DOE 217 and the Fresnel mirror 222 adopted for the fourthembodiment described above can be used not only in the plate-like imagepickup unit 207 but also in ordinary image pickup units, vari-focal lenselements for optical disks having different thicknesses, electronicendoscopes, TV cameras, film cameras and so on as shown in FIG. 60. Forchanging focal length more speedily, it is more preferable to use tolaneseries liquid crystals, for example DON-605: N−1 prepared by DainihonInk, Co., Ltd. (Monthly Report of Japanese Chemical Association.February 1997, p14 through p18) which has a high optical anisotropy(Δn=0.283; Δn represents an optical anisotropy which is a differencebetween principal axes of optical indicatrices) and a low viscocity.Such a liquid crystal permits changing a refractive index speedily,thereby making it possible to obtain optical elements having opticalcharacteristics which can be varied at higher response.

In the foregoing description, an intensity of a magnetic field ischanged mainly for varying an orientation of a liquid crystal. However,this method is not limitative and an orientation of a liquid crystal maybe varied by changing a frequency of an electric field. Further, anorientation of a liquid crystal may be varied by changing an intensityor a frequency of a magnetic field.

When an orientation of a liquid crystal is changed by varying afrequency of an electric field using a liquid crystal having adielectric anisotropy whose sign is changed by varying a frequency inparticular, it is possible to change a refractive index speedily,thereby obtaining an optical element which is capable of varying anoptical characteristic at high response.

Further, the following fact is applicable to all the optical elementshaving variable optical characteristics according to the presentinvention.

A substance having a refractive index which can be changed by varying anelectric field, a magnetic field, a temperature or the like can be usedin place of a liquid crystal. In other words, it is possible to form anoptical element having a variable optical characteristic by using amaterial of polymers in which a substance having a variable refractiveindex is dispersed. It is also preferable that an optical element havinga variable optical characteristic also satisfies any one, a combinationof certain ones or all of the conditions (18), (18-5), (19), (19-5),(1-1) and (1).

BaTiO₂ is known as an example of substance which has a refractive indexchanged by applying an electric field, lead glass and quartz are knownas examples of substances which have refractive indices changed byapplying a magnetic field, and water and the like are known as examplesof substances which have refractive indices changed by varying atemperature.

What is claimed is:
 1. Vari-focal spectacles comprising: a lens unitconfigured so that a lens element for spectacles has a plurality ofareas at least one of which has a vari-focal function; and a supportmember for setting at least said lens unit before an observer's eyeballsso as to permit saving a trouble of exchanging a plurality of spectaclesby changing a focal length of at least an area of said lens unit,wherein said lens element has at least an area having a fixed focallength and said plurality of areas having a fixed focal length areformed within a range of a surface of said lens element.
 2. Thevari-focal spectacles according to claim 1, wherein said lens unit hasthe area having the vari-focal function at least below a center of saidlens unit.
 3. The vari-focal spectacles according to claim 1, whereinsaid lens unit has an area having a fixed focal point at least above acenter of said lens unit.
 4. The vari-focal spectacles according toclaim 1, wherein said area are having the vari-focal function disposedin said lens unit is configured so as to contain a refractive indexvariable substance.
 5. The vari-focal spectacles according to claim 4,wherein said refractive index variable substance is configured in aperiodically arranged condition.
 6. The vari-focal spectacles accordingto claim 1, wherein said area having the vari-focal function disposed insaid lens unit is configured so as to contain a liquid crystalsubstance.
 7. The vari-focal spectacles according to claim 1, whereinsaid area having the vari-focal function disposed in said lens unit isconfigured so as to contain a liquid crystal substance and a diffractiveoptical element.
 8. The vari-focal spectacles according to claim 1,wherein said area having the vari-focal function is configured so as tocontain a liquid crystal substance and a polymer substance.
 9. Thevari-focal spectacles according to claim 1, wherein a partial area ofsaid lens unit having the vari-focal function consists of liquid crystalelements which are arranged in a spiral form satisfying the followingcondition: 2 nm≦P≦40λ wherein the reference symbols P represents aspiral pitch of molecules in said liquid crystal element and thereference symbol λ represents a wave length of rays in the twist nomaticcondition of said liquid crystal element.
 10. A lens unit configured sothat a lens element for spectacles has a plurality of areas at least oneof which has a vari-focal function, wherein said lens element has atleast an area having a fixed focal length and said plurality of areashaving a fixed focal length are formed within a range of a surface ofsaid lens element.
 11. The lens unit according to claim 10, wherein saidlens unit has an area having said vari-focal function at least below acenter of said lens unit.
 12. The lens unit according to claim 11,wherein said lens unit has an area having a fixed focal point at leastabove center of said lens unit.
 13. The lens unit according to claim 11,wherein said area having the vari-focal function disposed on said lensunit is configured so as to contain a refractive index variablesubstance.
 14. The lens unit according to claim 11, wherein said areahaving the vari-focal function disposed in said lens unit is configuredso as to contain a liquid crystal substance.
 15. The lens unit accordingto claim 11, wherein said area having the vari-focal function disposedin said lens unit is configured so as to contain a liquid crystalsubstance and a diffractive optical element.
 16. The lens unit accordingto claim 11, wherein said area having the vari-focal function disposedin said lens unit is configured so as to contain a liquid crystalsubstance and a polymer.
 17. The lens unit according to claim 11,wherein a refractive index variable substance contained in said areahaving the vari-focal function is configured in a periodically arrangedcondition.
 18. A vari-focal lens comprising: a lens unit having a lenselement; and a support member for setting said lens unit beforeobserver's eyeballs, wherein said lens element has a plurality of areaswithin a range of a surface including at least an area having a variablefocal length and at least an area having a fixed focal point.